The efficient, low cost, and low temperature processes used to make a variety of plastic articles have allowed organic polymer materials to replace glass in many technological applications. Cost aside, glass articles are preferred in most instances due to their superior physical properties. If it were possible to manufacture glass lenses or other parts by a low-cost process such as injection molding, such lenses and other parts would be competitive with their plastic counterparts.
Optical elements have various uses in many diverse technologies, including sensors, image projectors, displays (e.g., liquid crystal displays (LCDs), plasma display, and electro-luminescence display), as well as opto-electronic devices for telecommunications. As the telecommunications industry itself develops the need to develop precision optical elements that incorporate microstructures increases. In telecommunication devices, optical elements may be used, for example, in fiber and laser couplers, optical switches, or as diffraction gratings for WDM applications, and densely packed microlens arrays (MLAs) or networks for wavelength management modules or collimator applications. Precision optical elements require highly polished surfaces or exacting surface figures or qualities. The surfaces demand fabrication in proper geometric relationship to each other, and, where the elements are to be used in transmission applications, they will be prepared from a material of controlled, uniform, and isotropic refractive index.
Numerous methods and materials may be used to fabricate complex, precision optical elements. However, a great majority of conventional machining processes for manufacturing optical components are unsuited for producing very small features. Components having surface features or dimensions of 500 microns or smaller typically can be fabricated only through a few methods of limited applicability. The fabrication of microstructured surfaces using polymers is derived from processes developed by the semiconductor industry for making integrated circuits. For example, photolithography and ion etching techniques can be used to create structures having sub-millimeter surface features. However, these methods are not suitable for large-scale manufacturing. The process time required to etch a microstructure is proportionally dependent on the total depth of the microstructure. Moreover, not only are such methods expensive, but they can produce only a limited range of feature types. Etching processes are particularly worrisome because they can create rough surfaces. As a result, a smooth concave or convex profile, or true prismatic profiles, cannot be readily achieved using either of the two foregoing techniques.
Molding or hot embossing of plastics or glass materials, on the other hand, can form micron to sub-micron sized features. Plastics can conform to molds and reproduce faithfully intricate designs or fine microstructures. Unfortunately for many telecommunications applications, plastic materials are not ideal since they suffer from numerous shortcomings. For example, plastic materials have limited mechanical properties; are often not sufficiently robust to withstand, over time, environmental degradation; they exhibit large coefficients of thermal expansion (which can result in changes in volume and index of refraction); and it has been found that plastic optical devices often cannot withstand long exposure to humidity or high temperatures—all of which thereby limit the temperature range over which plastic optical devices may be useful.
Plastic materials also do not have complete transparency in the infrared (IR). As a result, one cannot use plastic materials to form a lens or other optical device in the IR range where the plastic absorbs. Further, plastics cannot transmit high-power light due to the internal heating of the material that will cause a plastic component's surface features to degrade and its index of refraction to change—both of which are unacceptable in an optical context. In addition, since plastic materials for optical applications are available in only a limited range of dispersion and refractive indices, plastics can provide only a restricted transmission range. As a result, the usefulness of plastic materials, even within the restricted telecommunications bandwidth, is limited by the tendency to accumulate internal stresses; a condition that results in distortion of transmitted light during use. Many plastics also scratch easily and are prone to yellowing or developing haze and birefringence; and the application of abrasive-resistant and anti-reflective coatings still has not fully solved these flaws for plastic materials. Finally, many chemical and environmental agents degrade plastics, which make them difficult to clean effectively.
In comparison to plastics, glass possesses properties that make them more suitable for use as an optical material. Glass normally does not suffer from the material shortcomings of plastics, and it can better withstand detrimental environmental or operational conditions. Hence, glass is a more preferred material, and glass optical components represent a different class of devices than those made from plastics. However, the requirements of the molding processes that use glass are more stringent than those for plastics.
Precision optical elements of glass are customarily produced by one of two complex, multi-step processes. In the first method a glass batch is melted at high temperatures and the melt is formed into a glass body or gob having a controlled and homogeneous refractive index. Thereafter, the glass body may be reformed using repressing techniques to yield a shape approximating the desired final article. The surface quality and finish of the body at this stage of production, however, are not adequate for image forming optics. As a result, the rough article is fine annealed to develop the proper refractive index and the surface features are improved by conventional grinding and polishing practices. In the second method the glass melt is formed into a bulk body, which is immediately fine annealed, cut and ground into articles of the desired configuration. Both of these methods have their limitations. Grinding and polishing are restricted to producing relatively simple shapes, for example, flats, spheres, and parabolas. Other shapes and general aspheric surfaces are difficult to grind and complicated to polish. On the other hand, conventional techniques for hot pressing of glass do not provide the exacting surface features and qualities that are required for clear image forming or transmission applications. The presence of chill wrinkles in the surface and surface features are unacceptable defects.
Glass molding has also traditionally presented a number of other problems. Generally, to mold glass one must use high temperatures, typically greater than about 700° C. or 800° C., so as to make the glass conform or flow into a requisite profile as defined by a mold. However, at such relatively high temperatures, glass becomes highly chemically reactive. Due to this reactivity, highly refractory molds with inert contact surfaces are required for molding glass: for example, molds of silicon carbide, silicon nitride, or other ceramic materials, intermetallic materials such as iron aluminides, and hard materials such as tungsten. However, in many cases such materials cannot attain sufficient surface smoothness for making satisfactory optical quality surface finishes. Precision optical elements require highly polished surfaces of exacting microstructure and quality. Metal molds can deform and recrystallize at high temperatures, both of which can adversely affect the surface and optical qualities of the article being molded. This means additional costs to repair and maintain the molds, and higher defect rates in the product. Second, also due to the reactivity of the glass at high temperatures, the molding often must be carried out in an inert atmosphere, which complicates the process and increases costs. Third, the potential for air or gas bubbles to be entrapped in the molded articles is another drawback of high-temperature molding. Gas bubbles trapped within glass articles degrade the optical properties of the articles. The bubbles distort images and generally disrupt optical transmission. Fourth, even at high temperatures, hot-glass molding cannot create intricate, high-frequency, submillimeter microstructures such as those required for diffraction gratings.
Molded glass articles have been described in the past. However, in previous processes, the molding is done by pressing (or compression molding) at ultra high viscosities in the range of 108-1012 poise. For example, an aspheric lens can be formed using these processes by pressing a glass gob in a mold at approximately 109 poise. Molding is carried out at these ultra-high viscosities in order to prevent crystallization of the glass.
The injection molding process as practiced by the polymer industry is typically run at a maximum temperature of 400° C. Therefore, in order for a glass to be injection molded, it is necessary that it have a 10,000 poise temperature of 400° C. or less and that it be resistant to crystallization when subjected to high shear rates of 1,000 to 10,000 sec−1 at its 10,000 poise temperature.
The use of chalcogenide glasses has been investigated for molding of glass elements having very fine surface features. However, these glasses typically contain arsenic. The inclusion of arsenic into the glass inhibits crystallization of chalcogenide glasses, which is necessary feature of hot-melt processing of chalcogenide glasses.
However, the use of arsenic has several disadvantages. During glass, production, manufacturers are exposed to arsenic, which is recognized as a human carcinogen (and has been implicated in lung cancer, skin cancers, stomach cancers, kidney and bladder cancers, and leukemia and lymphoma). Arsenic can also interfere with the production of adenosine triphosphate (ATP) in cells as well as increase the production of hydrogen peroxide, which in turn may lead to the production of reactive oxygen species and oxidative stress. Arsenic poisoning can ultimately result in death from the failure of multiple organs. Further, depending upon conditions of use, arsenic may leach from glass, creating an exposure hazard for users of optical devices manufactured from known chalcogenide glasses.
What is needed is an arsenic-free chalcogenide glass composition capable of being used in hot-melt processing techniques and equipment. Ideally, the composition would be stable against crystallization, transmissive to IR and near-IR radiation, and suitable for the fabrication of optical devices, including devices with fine or hyperfine microstructures.